The Precision Time Protocol (PTP) is a protocol utilized for synchronizing clocks throughout a network. PTP was originally defined by the Institute of Electrical and Electronics Engineers (IEEE) 1588-2002 standard, entitled “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” (hereby incorporated by reference), and released in 2002. In 2008, a revised standard, IEEE 1588-2008 entitled “IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” (hereby incorporated by reference) was released. This new version, also commonly known as PTP Version 2 (PTPv2), improves accuracy, precision, and robustness. IEEE 1588-2008 was initially utilized primarily by instrumentation and control applications. The telecom industry, however, has since adopted IEEE 1588-2008 as a solution for providing time synchronization for packet network applications such as mobile backhaul. These packet network applications, however, require microsecond or better time distribution over the network and high quality fault tolerance.
IEEE 1588-2008 defines a best master clock algorithm (BMCA), executed at each PTP clock in the PTP domain, for configuring the PTP clocks to be a grandmaster, boundary clock, or slave clock. By utilizing their BMCA to determine their roles in the PTP domain, the PTP clocks converge to form a synchronization tree. While the synchronization tree is being formed, all clocks start locking (i.e., synchronizing) to their masters. Typically, it requires several minutes for each PTP clock to synchronize to its master. After the synchronization tree is formed, if one or more of the PTP clocks or the network experience a fault, all the downstream clocks will rearrange/re-converge to form a new synchronization tree. During this re-convergence period, timing transients will occur as new master and slave ports are selected by the BMCAs. These timing transients result in a period of time where the clock quality advertised to the ultimate slave clocks is inaccurate. These timing transients persist until all PTP clocks in the synchronization tree actually acquire synchronization to their new masters. In other words, for several minutes, or perhaps an hour or longer in larger networks, the timing accuracy is not as good as advertised. This results in the application being provided with inadequate timing accuracy. The timing transients that occur during the re-convergence period are not acceptable by packet network applications in the telecom domain.
Known techniques, such as holdover, exist to work around certain failure/recovery scenarios. Such techniques, however, are inadequate for prolonged outages. Further, because the BMCA will advertise a clock quality that is not true during failure-induced rearrangements, holdover techniques will not work.